Vehicle airbags are safety devices that deploy toward the interior of a vehicle to help protect its occupants from injury in the event of a crash. Airbags may be concealed behind or beneath an interior panel during normal vehicle operation until such an event. When the airbag deploys, it typically does so through a deployment opening formed in or around the interior panel. The deployment opening may be pre-formed in the panel, the panel may move away to reveal the opening, or the opening may be formed during airbag deployment at a pre-determined location in the panel. Where formed during deployment, a tear seam may be provided in one or more components of the panel to control the location of material separation during airbag deployment. The tear seam usually consists of one or more stress concentrators formed in a layer of material along the desired tearing location. Notches, grooves, and other types of cuts in the material are examples of stress concentrators. Non-visible tear seams are sometimes desired and often constructed with the tear seam formed from the non-visible side of the material so that the stress concentrators are not apparent to the naked eye.
French Patent 2 875 562 to Godefroy describes a method of forming an airbag tear seam in a vehicle interior covering by perforating the covering from the visible side with a multi-needle piercing device. The device includes spring-loaded and retractable needles that extend from a housing and purports to allow the use of small needles that can help reduce the visibility of the holes produced by the needles. But the device's ability to produce a non-visible tear seam is questionable due to the large number of needles pressing simultaneously on the exterior of the covering, thus requiring a large load for penetration that can deform the surrounding area of the tear seam.